Blown freeze plug

Cooling problems over the many years but never blew same freeze plug before. 71 340, minor modes-intake, cam, roller rockers, headers. Running with no problems 4 years. Blew side freeze plug. Replaced, then blew after 2-3 miles. Figured not properly set flush, installed another, same results. Top hose, engine hot, bottom hose warm. Found kink in heater hose. When ahead and repaced, water pump and thermostat. Same, 1st 2 miles 190, then spikes 230, then back 190 with top hose, engine hot. Flushed engine both ways. Ideas?